[v2] performance: Improve ParseSigned whitespace stripping

[pquerna]

• When profiling some other code, I noticed ParseSigned wasn't as fast as I expected.
• Adds a benchmark for ParseSigned
• Replaces regexp to remove whitespace with a string.Builder approach.

benchcmp results for go test -v -run=NONE -bench BenchmarkParseSigned -benchtime=30s -benchmem

benchmark                  old ns/op     new ns/op     delta
BenchmarkParseSigned-4     8184          5461          -33.27%

benchmark                  old allocs     new allocs     delta
BenchmarkParseSigned-4     50             48             -4.00%

benchmark                  old bytes     new bytes     delta
BenchmarkParseSigned-4     3832          3656          -4.59%

[csstaub]

This is an excellent improvement, thank you @pquerna!

[mbyczkowski]

@csstaub any plans to port this to master/v3?